A new courtyard garden at the Royal Academy

Keeper's House at the Royal Academy is open daily for Friends of the Royal Academy until 4pm and then will open to the public until late; from October 2; Burlington House, Piccadilly, W1J 0BD; royalacademy.org.uk

An Islington garden Tom Stuart-Smith designed shows the striking effect of tree ferns in a sheltered plot

From Wednesday October 2, welcome an exciting new arts club in the heart of Mayfair, courtesy of the Royal Academy. Keeper's House will be open to Friends of the Royal Academy daily until 4pm, and then will open to the public until late.

Accessible from within Burlington House, the Royal Academy's home in Piccadilly, and from its famous courtyard, the new club, with architecture by Long & Kentish and interiors by David Chipperfield, features two lounges and an Oliver Peyton restaurant and bar over three floors, as well as new artwork by Tracey Emin and Grayson Perry.

However, arguably the club's most spectacular artwork will be its own courtyard, reached through the fire engine red cocktail bar: a dynamic green space with three-metre high Australian tree ferns that are even older than Keeper's House, built in the 1870s as a central London home for the Keeper of the Royal Academy.

Converting a small, dark, neglected basement housing an ugly substation into a beautiful social space is a tough call, but leading landscape designer Tom Stuart-Smith, who is used to transforming unpromising town plots as well as large country gardens, including those at Windsor Castle, relished the challenge.

"It's a tiny space, but I saw how you could make it like a little London garden, a patch of green," says Stuart-Smith, who believes that the smaller the space, the simpler it needs to be. "I decided to make more of what it already is, giving it an almost archeological quality like an excavation, using dark, narrow Flemish bricks for the steps and floor," he says.

Instead of the obvious, bringing in light with pale stone, which he says inevitably gets covered in algae, he brought in light with plants. "Tree ferns look iridescent, acting almost like light transmitters. Unlike many trees which make places darker, they bring light into a space in an extraordinary way. They also have a curious anthropomorphic quality and I like the idea of coming into a space that feels outlandish," he says.

On a practical level, tree ferns, in central London at any rate, stay green throughout the year, can be planted in a small level of soil provided the trunk is secured, and will thrive in a space that has no direct sunlight. Choosing tree ferns that have trunks with bends and curves was crucial, says Stuart-Smith. "The straight ones are incredibly boring and look like cigars popped into the ground. They would have looked awful," he says.

The usual trick with a small basement is to run planters all around the perimeter, freeing up the central space, but Stuart-Smith argues that when you do that, you immediately reduce the size. Instead, he planted the tree ferns directly into the ground at intervals throughout the garden, using shorter ones on the roof of the substation. "They're on three different levels, so people looking down on the garden from surrounding buildings will read it as one, and it will look like a green canyon," he says.

The planting needed to be robust, so Stuart-Smith surrounded each tree fern with the compact evergreen shrub Pittosporum tobira Nanum, as well as the lush green grass Hakonechloa macra, which will spread. The substation's walls are clothed with Virginia creeper and the city's favourite easy-going climber, Trachelospermum jasminoides.

"If you imagine 20 people sitting on the steps or on the low walls, they're not going to say, 'Oh, Pittosporum! What a lovely plant!' They're going to say, 'Tina, how lovely to see you!' and crunch, step on the Pittosporum."

The smaller the garden, the more lighting it needs, says Stuart-Smith, adding: "The key thing is never to be able to see the source. Aside from safety lighting on the steps, and a little lighting on the walls so people can see where they're going, we're uplighting the ferns so that each canopy will be a bright green light. The effect will be beautiful."